1. Field of the Invention
The present invention relates to an active noise control system which produces a signal that is interfere with and attenuates an uncomfortable confined engine noise generated in the passenger compartment of a vehicle by the operation of the engine, the signal being equal in amplitude and opposite in phase with the confined engine noise.
2. Description of the Related Art
The confined engine noise is a radiant noise which is generated by a vibrational force, created by the operation of the engine of a vehicle, being transferred to the vehicle body and thus causing resonance to occur in the passenger compartment or a closed space under a certain condition. Thus, the confined engine noise has noticeable periodicity in synchronization with the rotational speed or frequency of the engine.
A conventionally known active noise control system for reducing such uncomfortable confined engine noise adopts a method of providing feedforward adaptive control using an adaptive notch filter (e.g., see Japanese Laid-Open Patent Publication No. 2000-99037). FIG. 10 is a view illustrating the configuration of a conventional active noise control system disclosed in Japanese Laid-Open Patent Publication No. 2000-99037.
Referring to FIG. 10, a discrete computation for implementing the active noise control system is performed in a discrete-computation processor unit 17 such as a DSP (Digital Signal Processor). First, a wave shaper 1 removes noises or the like superimposed on an engine pulse while shaping the engine pulse. The resulting output signal from the wave shaper 1 is supplied to a cosine-wave generator 2 and a sine-wave generator 3, where a cosine wave and a sine wave are created as a reference signal. The reference cosine-wave signal or an output signal from the cosine-wave generator 2 is multiplied by a filter coefficient W0 of a first one-tap adaptive filter 5 in an adaptive notch filter 4. Similarly, the reference sine-wave signal or an output signal from the sine-wave generator 3 is multiplied by a filter coefficient W1 of a second one-tap adaptive filter 6 in the adaptive notch filter 4. The output signal from the first one-tap adaptive filter 5 and the output signal from the second one-tap adaptive filter 6 are added together at an adder 7, which in turn supplies the resulting output signal to a secondary noise generator 8. The secondary noise generator 8 produces a secondary noise, which is then interfere with and cancels the noise caused by the engine pulse. At this time, a residual signal that remains from the acoustic coupling in a noise suppressor portion is employed as an error signal “e” for use in an adaptive control algorithm.
On the other hand, at a notch frequency to be suppressed that is determined from the rotational frequency of the engine, the reference cosine-wave signal is supplied to a transfer element 9 having C0 that simulates the transfer characteristics between the secondary noise generator 8 and the noise suppressor portion. Likewise, the reference sine-wave signal is supplied to a transfer element 10 having C1 that simulates the transfer characteristics between the secondary noise generator 8 and the noise suppressor portion. The resulting output signals from the transfer element 9 and the transfer element 10 are added together at an adder 13 to produce a simulation cosine-wave signal r0, which is in turn supplied together with the error signal “e” to an adaptive control algorithm processor unit 15. The filter coefficient W0 of the adaptive notch filter 4 is successively updated in accordance with an adaptive control algorithm, e.g., the LMS (Least Mean Square) algorithm or a type of the steepest-descent method.
In the same manner, at the notch frequency to be suppressed that is determined from the rotational frequency of the engine, the reference sine-wave signal is supplied to a transfer element 11 having C0 that simulates the transfer characteristics between the secondary noise generator 8 and the noise suppressor portion. Likewise, the reference cosine-wave signal is supplied to a transfer element 12 having −C1 that simulates the transfer characteristics between the secondary noise generator 8 and the noise suppressor portion. The resulting output signals from the transfer element 11 and the transfer element 12 are added together at an adder 14 to produce a simulation sine-wave signal r1, which is in turn supplied together with the error signal “e” to an adaptive control algorithm processor unit 16. The filter coefficient W1 of the adaptive notch filter 4 is successively updated in accordance with an adaptive control algorithm, e.g., the LMS algorithm.
In this manner, the filter coefficients W0 and W1 of the adaptive notch filter 4 converge recursively to an optimum value so as to minimize the error signal “e,” i.e., to attenuate the noise in the noise suppressor portion.
However, in the aforementioned conventional active noise control system, since the characteristics of the secondary noise generator may vary with time or the environment in the passenger compartment may vary due to a window being opened or closed or an increase or decrease in the number of passengers, the present transfer characteristics between the output of the adaptive notch filter and the adaptive control algorithm processor unit may have changed from the previous transfer characteristics therebetween available upon determination of the characteristics of a transfer element simulating the previous transfer characteristics. Under these circumstances, the active noise control system may operate causing an unstable operation of the adaptive notch filter. This would not only make it difficult to provide an ideal noise reduction effect but also bring the system into divergence causing a noise to be further increased.
Furthermore, even under the circumstances where there exist a significant amount of incoming external noises while the vehicle is running on unpaved roads or a window is kept open, the system would not properly update the filter coefficients, thereby causing an unstable operation of the adaptive notch filter. In this case, at the worst, it is highly possible that divergence may occur to generate an abnormal acoustic noise causing the passenger to feel extremely uncomfortable. Moreover, in the presence of a difference between the noise level at the noise suppressor portion and that at the ears of a passenger, the system may cause an overcompensated condition in which noises are not properly attenuated at the ears of the passenger.